At present, we are experiencing a rapid integration of fixed and mobile networks, while IMS (IP Multimedia Subsystem) is generated under such environment. IMS is a global, independent on access and standard-based IP channel and service control architecture, which enables the terminal users based on ordinary internet protocol to use different types of multimedia services. IMS system not only offers a variety of access modes, but also provides inter-operations with the circuit switch domain. For a multi-mode mobile terminal currently locates in a packet switch domain, it may handover a conversation from a packet switch domain to a circuit switch domain to ensure the quality of the conversation, when it moves to an edge of or outside a network covered by the packet switch domain, or when it is located in a network covered by the circuit switch domain while the network covered currently by the packet switch domain becomes unavailable.
In prior art, if a user terminal can receive and transmit information over a packet switch domain and a circuit switch domain simultaneously, a handover is generally completed before the current connection is released. In other words, the user terminal establishes a call or session in the circuit switch domain and meanwhile maintains the existing voice call in the packet switch domain. The user handovers a voice media to the call or session of the circuit switch domain only after the call or session of the circuit switch domain is established, and then releases network resources in the packet switch domain before the handover. This manner usually has the shortest break duration. However, for the terminals which cannot receive and transmit information over two domains simultaneously, this method is inapplicable.
Therefore, 3GPP TS 23.216 specifies a SRVCC (Single Radio Voice Call Continuity) solution for maintaining voice call continuity between EPS (Evolved Packet System) PS (Packet Switch) access and UTRAN/GERAN (Universal Terrestrial Radio Access Network/GSM EDGE Radio Access Network) CS (Circuit Switch) access for calls that are anchored in IMS system. In this situation, UE (user equipment) is can merely transmit or receive data via one of the preceding two access networks at a given time.
However, SRVCC solution prescribed by the 3GPP TS 23.216 also contains some disadvantages, such as uncertain voice break duration, complex signaling flows, or the like. Why there is uncertain voice break duration will be described in detail in the following.
FIG. 1 illustrates SRVCC network architecture of a handover of VoIP conversation from E-UTRAN (Evolved UTRAN) to UTRAN/GERAN as prescribed by the 3GPP TS 23.216.
As shown in FIG. 1, UE accesses IMS via E-UTRAN and S-GW/PDN GW. The E-UTRAN is also referred to as LTE (Long Term Evolution), including a plurality of E-Node B in charge of the wireless access network part. EPS conducts a functional integration on NodeB, RNC (Radio Network Controller) and CN (Core Network) in the existing WCDMA and TD-SCDMA systems, and is simplified as two network elements, eNodeB and EPC. EPC comprises MME (Mobility Management Entity) for acting as a control node responsible of signaling processing of the core network, and S-GW (Serving Gateway)/PDN-GW (Packet Data Network Gateway) responsible of data processing of the core network. Wherein, non-3GPP wireless access network may access EPC via PDN-GW, and 3GPP wireless network may access EPC through S-GW.
In addition, FIG. 1 also illustrates interfaces between network elements suggested by the Specification. For example, E-UTRAN connects with EPC via a S1 (similar to lu) interface. E-UTRANs connect with each other via a X2 (similar to lur) interface (not shown), and UE connects with E-UTRAN via a LTE-Uu interface.
In the environment shown in FIG. 1. UE may decide to handover to the circuit switch domain provided by UTRAN/GERAN when it is located at an edge of the coverage of E-UTRAN or outside an area covered by E-UTRAN. In UTRAN/GERAN, the UE accesses the IMS network via a base station and a MSC (Mobile Switch Centre) server.
Wherein, UTRAN is a kind of relatively new access network for UMTS, and has now became an important access manner of UMTS, which includes NodeB, RNC, CN, etc. While GERAN is a key part of GSM specified and maintained by 3GPP and also be included in UTMS/GSM network, comprising base stations BSs, base station controllers BSCs and interfaces thereof (e.g. Ater interface, Abis interface, A interface, etc.). Commonly, the network of a mobile operator is composed of a plurality of GERANs, and combined with UTRAN in UMTS/GSM network.
Detailed information regarding the other network elements and the communication manners of the network elements in FIG. 1 can refer to TS23.216.
FIG. 2 shows a call flow for a handover of SRVCC from E-UTRAN to UTRAN/GERAN without DTM/PSHO (Dual Transfer Mode/Pack Switch Handover) support as prescribed by 3GPP TS 23.216. In order to accomplish the handover of voice conversation, the voice conversation needs to be anchored beforehand in IMS, such as on a SCC AS (Service Centralization and Continuity Application Server).
As shown in FIG. 2, when source E-UTRAN decides to perform a handover from a packet domain to a circuit domain on a ongoing VoIP call of a local UE in according to a measurement report received from local/source UE, it sends a handover request to local MME, and then the source MME splits the bearer (used for transferring the voice service subsequently), and sends a corresponding request of handover from the packet domain to the circuit domain to the MSC server or media gateway capable of covering the local UE currently. The corresponding MSC/media gateway initiates a session transfer after a handover preparation and setting up a circuit. Here, it should be noted that, if the target MSC to which the local UE is to be handover and the MSC that received the handover request from the MME are the same MSC, steps 6, 8 and 9 in the dashed part could be omitted (so as steps 20 and 21).
Next, SRVCC contains a session handover procedure at IMS layer and a cell handover procedure to the target cell at layer 2. That is to say, there are two user-plane handovers in the SRVCC.
1) Steps 10 to 12, SCC AS in IMS executes a session handover procedure, updates remote UE (i.e. the counter party that establishes the VoIP conversation with the local UE) with the SDP of the target CS access leg, and releases the source EPC PS access leg. These steps will result in a switch of the voice component of the ongoing conversation from EPC to MGW on user-plane.
2) Steps 15 to 21, a handover from E-UTRAN to UTRAN/GERAN is executed on the local UE and access network. This is a handover between RATs (Radio Access Type) performed at the local UE and access network, and will result in a handover of the local UE from the current E-UTRAN cell to the target UTRAN/GERAN cell.
VoIP call break, namely, voice flow break, will be generated in both of the above two handover procedures. Although each step in FIG. 2 is numbered with successive numbers, however, it does not mean the relationship on time between steps 10-12 and steps 15-21. On contrary, it is entirely possible that steps 10-12 are executed after step 15, or at the same time with step 15. In other words, there is no synchronization between the two handover procedures, which makes the voice break duration become uncontrolled and is uncertain. In the worst case, the voice break duration could be extreme long and deteriorates users' experiences.
FIG. 3 illustrates the duration of the break of VoIP conversation in the above SRVCC solution. Wherein, T1 represents the break duration of the cell handover procedure at layer 2, and meanwhile indicates the start time and finish time of the break. T2 represents the break duration of the session handover procedure at IMS layer, and meanwhile also indicates the start time and finish time of the break.
Case 1: the break caused by the session handover procedure at IMS layer happens before the break caused by the cell handover procedure. In this case, the break duration generated by SRVCC will greater than the maximum value of T1 and T2, with no greater than T1+T2, and in the worst case, equal to T1+T2.
Case 2: the break caused by the session handover procedure at IMS layer happens at the same time as the break caused by the cell handover procedure. In this case, the break duration generated by SRVCC is equal to the maxi value of T1 and T2.
Case 3: the break caused by the cell handover procedure happens before the break caused by the session handover procedure at IMS layer. In this case, similar as the Case 1, the break duration generated by SRVCC will greater than the maximum value of T1 and T2, with no greater than T1+T2, and in the worst case, equal to T1+T2.
It can thus be seen that the break duration generated in the SRVCC solution proposed in 3GPP TS 23.216 Specification is inconstant and uncontrollable.